The present invention relates in general to vehicular air conditioning systems with variable displacement compressors, and, more specifically, to optimizing an increased cooling during a stop event so that passenger comfort is maintained during a stopped condition of the vehicle with the engine also stopped.
Fuel economy of automobiles is an important attribute of vehicle performance, and is determined by the technology employed in its design, by driver behavior and actions, and by conditions under which the vehicle is used (e.g., speed, road design, weather, and traffic). Manufacturers continuously strive to deliver better fuel economy. One technology being increasingly used is Stop-Start, wherein an internal combustion engine automatically shuts down when the vehicle comes to a stop and then restarts as needed to reduce the amount of time the engine spends idling (e.g., while waiting at a traffic light), thereby improving fuel economy and reducing emissions. Start-Stop technology can provide a 5% to 10% improvement in fuel economy.
In addition to vehicle propulsion, the engine drives other vehicle systems such as an air conditioning compressor. Occupant comfort must be maintained during the time that the engine is stopped. Since the air conditioning compressor runs on a front-end-accessory-drive (FEAD) belt driven by the engine, the compressor does not run when the engine is stopped. Thus, when the air conditioning system is actively being used and the engine stops during a vehicle stop, the cooling action is interrupted and the passenger cabin may become warmer. If the temperature increases by a certain amount, the engine is usually restarted so that cooling resumes, but some of the fuel economy improvement is lost.
For improved efficiency and performance, many vehicles now use variable displacement compressors. By varying the displacement of the compressor (e.g., by adjusting the piston stroke) to match the cooling needed at any particular time, the engine load is reduced when cooling demand is lower. The stroke can be electrically controlled via a swash plate, for example, within a range of 0% to 100% of the maximum stroke. By appropriately controlling the stroke, fuel economy improvements are realized during running of the engine.
In connection with the use of a variable displacement compressor in a vehicle with Start-Stop technology, it has been recognized that it can be beneficial to increase the stroke during coasting of the engine so that energy in the coasting engine that would otherwise be wasted can be regenerated into extra cooling. This allows the compressor to temporarily run at a lower stroke once the engine restarts. Examples of this regenerative cooling include U.S. Pat. No. 7,484,375 to Engel and U.S. Pat. No. 4,425,765 to Fukushima et al.
In known systems, however, a maximum cooling effect is obtained during coasting which leads to disadvantages. Unless a cold storage means is provided in the evaporator, the cooling of the passenger cabin may become uncomfortable. A sufficient cold storage capacity to avoid discomfort may be high. Moreover, many vehicles with Start-stop do not stop running during vehicle coasting, but only stop once the vehicle has come to a full stop. Therefore, the increased compressor stroke would actually increase engine load (and fuel consumption) during coasting.
It would be desirable to optimize the amount of extra cooling obtained during the slowing down and stopping of a vehicle equipped with a variable displacement compressor and start-stop technology. As a result, the time delay before the engine must be restarted during a vehicle stop due to increasing cabin temperature is optimized.